wal

WAL stands for write-ahead logging, a database journaling technique used to ensure durability and atomicity of transactions. In a WAL system, changes to the database state are written to a log before they are applied to the actual data files. The log is stored on stable storage, and the data pages are updated after the log records are durably written. If a crash occurs, the log can be replayed to recover the database to a consistent state by redoing committed changes and undoing partial ones.

Operation and recovery: When a transaction modifies data, a log record describing the change is written and flushed to the log before the data pages are updated. The system can acknowledge commits only after the relevant log has been durably stored. During recovery, the log is read sequentially to reconstruct the state of the database, applying committed updates and discarding incomplete work from failed transactions.

Usage and benefits: WAL is integral to many modern database systems and is central to crash recovery, point-in-time recovery, and replication. In practice, databases such as PostgreSQL use WAL to stream changes to standby servers for physical replication and to archive logs for backups. WAL segments can be archived and replayed to restore data to a specific moment in time. Checkpoints and log shipping are common mechanisms associated with WAL-based architectures.

Variants and considerations: Some systems implement WAL-like mechanisms with different terminology or modes, such as SQLite’s write-ahead logging mode, which uses a separate log file to optimize certain workloads. Effective use of WAL requires consideration of durability settings, backup strategies, and log management to ensure reliable recovery.